It would help to know more about your system setup. If you run "inxi -Fxzd" from the console terminal prompt, highlight the results, copy and paste them back here, that should provide enough information.

That is unusual. Did you change anything recently, or install something new, if so what?

As is typical with KDE, there are various options and settings. Desktop themes have a lot to with this. And, there is a separate "translucency" option in Desktop Effects with its own settings in the System Settings menu. In Linux Mint 18.x KDE System Settings -> Desktop Behavior -> Desktop Effects ->Translucency.